To further increase the magnetic recording density of a hard disk drive (HDD) system, there have been growing demands for the improvements in the performance of thin film magnetic heads. A perpendicular magnetic recording (PMR) head combining a single pole writer with a tunneling magnetoresistive (TMR) reader provides a high writing field and a large read-back signal, thus a higher areal density can be achieved. Increasing the magnetic recording areal density requires smaller grain size in the magnetic recording media, which in turn reduces storage lifetime due to thermal instabilities. In order to maintain durable storage lifetime, the thermal stability (energy barrier Δ=KV/kbT) has to be increased. In the formula, kbT is the Boltzmann constant, T is the temperature in Kelvin, V is the average grain size of the storage media, K, magnetic anisotropy, is equal to HkMs/2, where Hk is the magnetic anisotropy field and Ms is the saturation magnetization of the magnetic recording media. For smaller grain size media, K has to be increased to maintain the same thermal stability for storage, thus Hk is greatly increased causing the magnetic media to have high coercivity Hc. As a consequence, the magnetic field generated by the magnetic writer main pole as well as the current from the coil around the main pole may not be strong enough to switch the magnetic media bits for data recording.
To solve this magnetic recording dilemma, thermally-assisted magnetic recording (TAMR) has been introduced. The purpose of TAMR is to use heat energy to reduce the energy barrier (Δ=KV/kbT) of the grains of the magnetic recording media while writing the data with the magnetic recording field. During data writing, a magnetic bit in the media is heated, causing temperature to rise; then the bit in the media can be reversed in polarity by the applied magnetic field due to a reduction in the energy barrier Δ. Once the bit polarity is changed, both the heating source and the applied field are quickly withdrawn. As the temperature reverts to room temperature, the switched state of the bit is stored in the magnetic recording media.
In present thermally-assisted magnetic recording (TAMR), the heating source is produced by the means of near-field optical radiation. The near-field optical radiation is produced by plasmons excited by irradiation of the light in a metal layer. In TAMR, a laser beam, generated by a laser diode is transmitted through a waveguide at whose distal air bearing surface (ABS) end it couples to a planar plasmon generator (PPG) formed of a conducting metal film surrounded by a dielectric material. Such metal films are able to generate near-fields efficiently by the excitation of surface plasmons (SP), which are resonant surface modes of free electrons bounded within the metal-dielectric interface. Structure and geometry of the PG can be engineered to enable efficient energy transfer from waveguide to PG, to excite local surface plasmon resonance, and to utilize the “lightning rod” effect to further improve field confinement. This kind of metallic nanostructure is referred to as the planar plasmon generator (PPG) or near-field transducer (NFT). The nature of the near-fields is such that they are not subject to diffraction effects and can be focused to a very small spot size where they heat the magnetic media. Typically, the metals used for the PPG are noble metals such as gold, silver, and other highly conductive metals such as copper, and their alloys, due to the large availability of free electrons and the low optical absorption in those metals.
The PPG materials not only need to generate surface plasmons with high efficiency, but they also have to be reliable under high temperature irradiation during TAMR writing process. Under such high temperature irradiation, the materials have to adhere firmly to their dielectric surroundings and do so without any deformations during long time writing processes with heating. The PPG film has to be reliable and durable for multiple TAMR writer processes. The combined requirements of high conductivity, reliability and durability has led to a continuing search for better materials for the PPG structure.